UserForms stylesheets are written in SASS, so to make changes you'll need to have Compass installed.
$ apt-get update
$ apt-get install ruby
$ gem install compass
Install homebrew. Then:
$ brew update
$ brew install ruby
$ gem install compass
Make your changes to scss/UserForms.scss
or scss/UserForms_cms.scss
. Then navigate to the userforms
folder and run:
$ compass compile